This post is meant to provide information for those who don't already know. If you already knew this information than please spare all of us the posts "I could have told you that"........So I have an A&K S system M4 and last month the two tabs broke on the back of the upper receiver. I looked and looked for an S system upper receiver but couldn't find one in stock any where. Finally I found one at ehobbyasia. For those who aren't aware the front of the upper receiver is different for the S System. It took about 15 days to get to AZ from Hong Kong. I received the part and noticed that it looked nothing like the picture on the web site. the front was threaded like the regular M4 upper. Upon closer inspection i noticed two very small screws . You can actually take the threaded part out of the upper receiver and replace it with the s system part (didn't know that).
when you put it back together the back of the upper receiver does not match up perfectly so you may have to drill out the tabs a bit to insert the pin. I'm just glad I was able to fix it.
P.S. I tried to post photos of what I am talking about but I'm too stupid to figure it out, and don't say "click the insert photo button" that does not work
